In some countries the tax year coincides with the calendar year, however for us brits this is not the case. To understand the reasoning behind the UK’s tax year start date of 6thApril you have to go back to medieval times.
In England and Ireland the New Year used to start on 25th March also known as ‘Lady Day’ (the commemoration of the angel Gabriel’s announcement to the Virgin Mary that she would become he mother of Jesus Christ) this was one of the most important days in the religious calendar. The other important dates were Midsummer on 24th June, Michaelmas on 29th September and Christmas day 25th December. All accounts including debts and rents had to be settled by these days and as Lady Day was the first this gradually became regarded as the start of the financial year.
The move forward to April 6 resulted from changes to the calendar and the number of days in various years. Despite the Julian calendar working well for centuries in Europe, it did not align with the solar calendar which caused problems over time, in particular with the Roman Catholic Church as the celebration of Easter had been gradually getting later. By 1752 England was the only European country to still be using the Julian calendar making it 11 days out of alignment with the rest of Europe. To tackle this issue 11 days were dropped from September and to ensure that there was no loss of tax revenues the Treasury extended the 1752 tax year by adding these 11 days at the end. Consequently the beginning of the 1753 tax year was moved to 5thApril.
In 1800, the start of the tax year was moved to the 6th April to mitigate for the differences between the Julian calendar and Gregorian calendar. Ever since this has remained the date of the beginning of the tax year for the UK, becoming formalised in 1900.